Partitioning

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Revision as of 10:52, 19 February 2013 by Bwid(talk | contribs)(2 MiB is unnecessary large. grub 2 manual recommends "larger than the bare minimum, such as 1MiB". Making it 1007 KiB will make the next partition aligned at 1024 KiB on a GPT disk.)

Partitioning a hard drive allows one to logically divide the available space into sections that can be accessed independently of one another. Partition information is stored within a hard drive's GUID Partition Table or Master Boot Record.

An entire hard drive may be allocated to a single partition, or one may divide the available storage space across multiple partitions. A number of scenarios require creation multiple partitions: dual- or multi-booting, for example, or maintaining a swap partition. In other cases, partitioning is used as a means of logically separating data, such as creating separate partitions for audio and video files. Common partitioning schemes are discussed in detail below.

Users may create up to four primary partitions per hard drive (in case of MBR). If additional partitions are required, a single extended partition can be created instead (that is, up to three primary partitions and one extended partition). An extended partition can be further divided into an unlimited number of logical partitions.

Partition type

Partitioning a hard disk drive defines specific memory storage areas. These are called partitions. Each partition behaves as a separate disk and is formatted with a specific filesystem type (see below).

On MBR

There are 3 types of disk partitions:

Primary

Extended

Logical

Primary partitions can be bootable and are limited to four partitions per disk or RAID volume. If a partitioning scheme requires more than four partitions, an extended partition containing logical partitions is used. Extended partitions can be thought of as containers for logical partitions. A hard disk can contain no more than one extended partition. The extended partition is also counted as a primary partition so if the disk has an extended partition, only three additional primary partitions are possible (i.e. three primary partitions and one extended partition). The number of logical partitions residing in an extended partition is unlimited. A system that dual boots with Windows will require that Windows reside in a primary partition.

The customary numbering scheme is to create primary partitions sda1 through sda3 followed by an extended partition sda4. The logical partitions on sda4 are numbered sda5, sda6, etc.

On GPT

There is only one type of partition, Primary. The amount of partitions per disk or RAID volume is unlimited.

Partition scheme

There are no strict rules for partitioning a hard drive, although one may follow the general guidance given below. A disk partitioning scheme is determined by various issues such as desired flexibility, speed, security, as well as the limitations imposed by available disk space. It is essentially personal preference. If you would like to dual boot Arch Linux and a Windows operating system please see Windows and Arch Dual Boot.

Single root partition

This scheme is the simplest and should be enough for most use cases. A swapfile can be created and easily resized as needed. It usually makes sense to start by considering a single / partition and then separate out others based on specific use cases like raid, encryption, a shared media partition, etc.

Discrete partitions

Separating out a path as a partition allows for the choice of a different filesystem and mount options. In some cases like a media partition, they can also be shared between operating systems.

Mount points

The following mount points are possible choices for separate partitions, you can make your decision based on actual needs.

/ (root)

The root directory is the top of the hierarchy, the point where the primary filesystem is mounted and from which all other filesystems stem. All files and directories appear under the root directory/, even if they are stored on different physical devices. The contents of the root filesystem must be adequate to boot, restore, recover, and/or repair the system. Therefore, certain directories under/are not candidates for separate partitions.

The / partition or root partition is necessary and it is the most important. The other partitions can be replaced by it.

Warning: Directories essential for booting must be on the same partition as / or mounted in early userspace by the initramfs. These essential directories are: /bin, /etc, /lib, /sbin and /usr[1].

/boot

The /boot directory contains the kernel and ramdisk images as well as the bootloader configuration file and bootloader stages. It also stores data that is used before the kernel begins executing user-space programs. /boot is not required for normal system operation, but only during boot and kernel upgrades (when regenerating the initial ramdisk).

A separate /boot partition is needed if installing a software RAID0 (stripe) system.

/home

Separating out /home allows / to be re-partitioned separately, but note that you can still reinstall Arch with /home untouched even if it isn't separate - the other top-level directories just need to be removed, and then pacstrap can be run.

You should not share home directories between users on different distributions, because they use incompatible software versions and patches. Instead, consider sharing a media partition or at least using different home directories on the same /home partition.

/var

The /var directory stores variable data such as spool directories and files, administrative and logging data, pacman's cache, the ABS tree, etc. It is used, for example, for caching and logging, and hence frequently read or written. Keeping it in a separate partition avoids running out of disk space due to flunky logs, etc.

It exists to make it possible to mount/usras read-only. Everything that historically went into/usrthat is written to during system operation (as opposed to installation and software maintenance) must reside under/var.

Note:/var contains many small files. The choice of filesystem type (see below) should consider this fact if a separate partition is used.

/tmp

This is already a separate partition by default, by virtue of being mounted as tmpfs by systemd.

Swap

A swap partition provides memory that can be used as virtual RAM. A swapfile should be considered too, as they have almost no performance overhead compared to a partition but are much easier to resize as needed. A swap partition can potentially be shared between operating systems, but not if hibernation is used.

Note: The old rule of matching the swap partition size with the available RAM when using suspend-to-disk no longer applies. The default suspend method uses an image the size of 40% of the currently available RAM by default. Even with TuxOnIce the atomic copy generally only takes about 70% after compression.[2]

How big should my partitions be?

The size of the partitions depends on personal preference, but the following information may be helpful:

/boot — 100 MB

A /boot partition requires only about 100 MB.

/ — 15-20 GB

The root filesystem (/) must contain the /usr directory, which can grow significantly depending upon how much software is installed. 15-20 GB should be sufficient for most users with modern hard disks.

/var — 8-12 GB

The /var filesystem will contain, among other data, the ABS tree and the pacman cache. Keeping cached packages is useful and versatile as it provides the ability to downgrade. As a result, /var tends to grow in size. The pacman cache in particular will grow as the system is expanded and updated. It can, however, be safely cleared if space becomes an issue. 8-12 GB on a desktop system should be sufficient for /var, depending on how much software will be installed.

/home — [very large]

The /home filesystem is typically where user data, downloads, and multimedia reside. On a desktop system, /home is typically the largest filesystem on the drive by a large margin.

swap — [varies]

Historically, the general rule for swap partition size was to allocate twice the amount of physical RAM. As computers have gained ever larger memory capacities, this rule has become deprecated. On machines with up to 512MB RAM, the 2x rule is usually adequate. If a sufficient amount of RAM (more than 1024MB) is available, it may be possible to have a smaller swap partition or even eliminate it. With more than 2 GB of physical RAM, one can generally expect good performance without a swap partition.

Note: If available, an extra 25% of space added to each filesystem will provide a cushion for future expansion and help protect against fragmentation.

Partition Alignment

High-level Overview

Proper partition alignment is essential for optimal performance and longevity. The key to alignment is partitioning to (at least) the EBS (erase block size) of the SSD.

Note: The EBS is largely vendor specific; a Google search on the model of interest would be a good idea! The Intel X25-M for example is thought to have an EBS of 512 KiB, but Intel has yet to publish anything officially to this end.

Note: If one does not know the EBS of one's SSD, use a size of 512 KiB. Those numbers are greater or equal than almost all of the current EBS. Aligning partitions for such an EBS will result in partitions also aligned for all lesser sizes. This is how Windows 7 and Ubuntu "optimize" partitions to work with SSD.

If the partitions are not aligned to begin at multiples of the EBS (512 KiB for example), aligning the file system is a pointless exercise because everything is skewed by the start offset of the partition. Traditionally, hard drives were addressed by indicating the cylinder, the head, and the sector at which data was to be read or written. These represented the radial position, the drive head (= platter and side) and the axial position of the data respectively. With LBA (logical block addressing), this is no longer the case. Instead, the entire hard drive is addressed as one continuous stream of data.

Using GPT - Modern Method

GPT is an alternative, contemporary partitioning style. It is intended to replace the old Master Boot Record (MBR) system. GPT has several advantages over MBR, which has quirks dating back to MS-DOS times. With recent developments to the formatting tools fdisk (MBR) and gdisk (GPT), it is equally easy to use GPT or MBR and get maximum performance.

A special exception to this rule: dual-booting Windows Vista/7 64 bit, and using UEFI instead of BIOS, one must use GPT.

If none of the above apply, choose freely between GPT and MBR. Since GPT is more modern, it is recommended in this case.

Gdisk Usage Summary

The GPT-able tool equivalent to fdisk, gdisk, can perform partition alignment automatically on a 2048 sector (or 1024KiB) block size base which should be compatible with the vast majority of SSDs if not all. GNU parted also supports GPT, but is less user-friendly for aligning partitions. A summary of the typical usage of gdisk:

If the SSD is brand new or if wanting to start over, create a new empty GUID partition table (aka GPT) with the Template:Keypress command.

Create a new partition with the Template:Keypress command (primary type/1st partition).

Assuming the partition is new, gdisk will pick the highest possible alignment. Otherwise, it will pick the largest power of two that divides all partition offsets.

If choosing to start on a sector before the 2048th gdisk will automatically shift the partition start to the 2048th disk sector. This is to ensure a 2048-sectors alignment (as a sector is 512B, this is a 1024KiB alignment which should fit any SSD NAND erase block).

Use the +x{M,G} format to extend the partition x megabytes or gigabytes, if choosing a size that is not a multiple of the alignment size (1024kiB), gdisk will shrink the partition to the nearest inferior multiple.

Select the partition's type id, the default, Linux/Windows data (code 0700), should be fine for most use. Press Template:Keypress to show the codes list. If planning to use LVM select Linux LVM (8e00).

Warning: If planning to use the GPT partitioned SSD as a boot-disk on a BIOS based system (most systems except Apple computers and some very rare motherboard models with Intel chipset) one may have to create, preferably at the disk's beginning, a 1007 KiB partition with no filesystem and with the partition type as BIOS boot or bios_grub partition (gdisk type code EF02) for booting from the disk using GRUB. For Syslinux, one does not need to create this bios_grub partition, but one needs to have separate /boot partition and enable Legacy BIOS Bootable partition attribute for that partition (using gdisk). See GPT for more information.

Warning: GRUB legacy does not support GUID partitioning scheme, users must use burg, GRUB or Syslinux.

Warning: If planning to dual boot with Windows (XP, Vista or 7) do NOT use GPT since they do NOT support booting from a GPT disk in BIOS systems! Users need to use the legacy MBR method described below for dual-boot in BIOS systems! This limitation does not apply if booting in UEFI mode and using Windows Vista (64bits) or 7 (64bits). For 32-bit Windows Vista and 7, and 32 and 64-bit Windows XP, users need to use MBR partitioning and boot in BIOS mode only.

Using MBR - Legacy Method

Using MBR, the utility for editing the partition table is called fdisk. Recent versions of fdisk have abandoned the deprecated system of using cylinders as the default display unit, as well as MS-DOS compatibility by default. The latest fdisk automatically aligns all partitions to 2048 sectors, or 1024 KiB, which should work for all EBS sizes that are known to be used by SSD manufacturers. This means that the default settings will give you proper alignment.

Note that in the olden days, fdisk used cylinders as the default display unit, and retained an MS-DOS compatibility quirk that messed with SSD alignment. Therefore one will find many guides around the internet from around 2008-2009 making a big deal out of getting everything correct. With the latest fdisk, things are much simpler, as reflected in this guide.

Fdisk Usage Summary

Start fdisk.

If the SSD is brand new, create a new empty DOS partition table with the Template:Keypress command.

Create a new partition with the Template:Keypress command (primary type/1st partition).

Use the +xG format to extend the partition x gigabytes.

Change the partition's system id from the default type of Linux (type 83) to the desired type via the Template:Keypress command. This is an optional step should the user wish to create another type of partition for example, swap, NTFS, LVM, etc. Note that a complete listing of all valid partition types is available via the Template:Keypress command.

When finished, users may format their newly created partitions with mkfs.x /dev/sdXN where x is the filesystem, X is the drive letter, and N is the partition number.
The following example will format the first partition on the first disk to ext4 using the defaults specified in /etc/mke2fs.conf:

# mkfs.ext4 /dev/sda1

Warning: Using the mkfs command can be dangerous as a simple mistake can result in formatting the WRONG partition and in data loss! TRIPLE check the target of this command before hitting the Enter key!